- Email: [email protected]
Cross section measurements for the (3He, p) nuclear reaction on B and N between 2 and 4 MeV
__ i!B
2&k __
Nuclear Instruments and Methods in Physics Research B I I8 (19%) 2 19-223
Rl0NlB
Beam Interactions with Materials A Atoms
ELSEVIER
Cross
section measurements for the ( 3He, p) nuclear reaction on B and N between 2 and 4 MeV
*,
L.C. McIntyre Jr. aV J.A. Leavitt a, M.D. Ashbaugh a, J. Borgardt a, E. Andrade b, J. Rickards b, A. Oliver b a Depurtment ofPhysics. University ofAri:ona. Tucson. AZ 85721, USA b Institute de Fisica, U.NA.M..
Ap. Postal 20364, Mexico. D.F. 01000. Mexico
Abstract Cross section measurements have been made for several proton groups from the t3He,p) nuclear reaction on boron and nitrogen. Incident energies between 2 and 4 MeV and angles of 90” and 135” were used. A 1.5 mm thick surface barrier detector, covered to stop elastically scattered 3He ions, was used to detect the high energy protons from these reactions. The proton energy was as high as 20 MeV in the case of a boron target. We propose the use of these reactions for quantitative elemental areal density determination in thin film analyses.
1. Introduction One of the shortcomings of thin film analysis using Rutherford Backscattering Spectrometry (RBS) with MeV ion beams is the insensitivity to light elements in cases where interfering signals are present due to heavier elements in the film or backing substrate. In many cases the use of nuclear reactions is a viable alternative. Historically the (d, p) reaction is one of the most popular [I]; however, many materials analysis accelerator labs cannot provide adequate radiation shielding for the operating personnel and are reluctant to accelerate deuterons. In some cases, the (3He, p) or c3He, a> reactions can serve as alternatives with a reduced but not negligible radiation hazard. The use of these reactions in the analysis of C and 0 has been reported in several papers [2-71. There are two basic materials analysis techniques using nuclear reactions; one is the use of sharp resonances to depth profile element concentrations, the other is the use of regions of constant cross section to measure total elemental areal densities in thin films. In this paper, we present cross section measurements for several proton groups from the c3He, p) reaction on natural boron and nitrogen using incident energies between 2 and 4 MeV. In all cases the cross sections were found to be smoothly varying with energy: no resonance structure is evident. We have recently reported investigations concerning the use of the (o, p) nuclear reaction for determination of
Corresponding author. Tel. + 1 520 621 6813, fax + I 520 621 4721, e-mail [email protected]. l
B and N in thin films [8,9]. While this reaction can be used successfully in both cases, there are some problems which are not present if the corresponding c3He, p> reaction is IO used. Cross sections for the (a, p) reaction on B, ’ 'B and 14N all show evidence of resonance behavior in the regions explored. However, the resonances are too broad to be used for depth profiling and simply limit the regions of smooth or constant behavior of the cross section. This is particularly true in the case of B [8]. An additional problem with the N (a, p) reaction is the low Q value and possible interference with competing reactions if Si is present [9]. An advantage of the c3He, p) reactions on both boron isotopes and 14N is the large Q value. This results in high energy reaction protons with little chance of overlap by interfering reactions. The Q values for the c3He, p) reaction on “B, “B, and 14N are + 19.7 MeV, + 13.2 MeV, and + 15.2 MeV respectively. The downside is that a detector thick enough to stop these high energy reaction protons is required for optimal results. If a thinner detector is used, these high energy protons will lose some fraction of their energy in the detector and perhaps overlap lower energy particles that have been stopped with complete energy deposition. Thick absorber foils can be used to slow the reaction protons; however, straggling reduces the energy resolution significantly. Since we will not consider depth profiling in this paper, the energy resolution is of concern only for cleanly resolving proton groups for total areal density determination. A recent study was published using the 14N (“He, p) I60 reaction to determine N [IO]; however, only energies below 2.8 MeV were used and the ground state proton
0168.583X/%/$15.00 Copyright 6 1996 Elsevier Science B.V. All rights reserved SSDf 0168-583X(95)01074-2
III. NRA
220
L.C. McIntyre Jr. et al./Nucl. Instr. and Meth.
group was not detected available.
because a thick detector
in
Phyr. Res. B I18 (1996) 219-223
was not
2. Experimental
5
2.1. General
2
‘OB(P,) 0.9 -
E.
Beams of 3He+ with energies between 2 and 4 MeV were obtained from the University of Arizona CN (High Voltage Engineering Carp) Van de Graaff accelerator. A 1.5 mm (nominal) thick Si surface barrier detector with area 300 mm2 (nominal) was positioned approximately 50 mm from the target at laboratory angles of 90” or 135”. The target normal was 45” relative to the beam direction, and the angular acceptance range was + / - 8” about the central angle for both detection angles. The detector was covered with a 23 p,rn thick Mylar foil to stop elastically scattered ‘He+ ions. The nominal thickness of the Ortec surface barrier detector (model BA-023-300-1500) was 1.5 mm; however, since we observe linear behavior for incident protons up to 20 MeV the thickness must be at least 2 mm when operated with the prescribed voltage of 300 V. The solid angle of the detector was determined by a circular collimator and was measured to be 69 +/ - 4 msr. Typical beam currents were 80 nA and a typical run had an integrated charge of 20 KC. This thick detector was added to our usual backscattering setup [ 111:nuclear reaction and elastic scattering data can be taken simultaneously if desired. 2.2. Boron Fig. 1 shows a partial energy spectrum of reaction roducts at a lab angle of 135” from incident 3000 keV P about 2.6 X lOI He+ ions on a target containing
Epw 7
9
11
13
15
17
19
,.,,
@-ie,p)C
I
250
350
I
Ebeam= 3000 keV
I
9 = 135”
450
550
650
750
650
950
Channel Number
Fig, I. Partial energy spectrum of reaction products at a laboratory angle of 135” from 3000 keV incident 3He ions on a natural boron target on a Cu Backing. Tbe integrated charge was 20 KC.
B
0.7 -
;; D
:
2cco
.
0.
_..**.*
.*..-,
.**
2500
30x
3500
4000
Energy (keV)
Fig. 2. Measured laboratory differential cross sections for the indicated proton groups from the c3He, p) reaction on the isotopes of boron at a laboratory angle of 90”.
atoms/cm* of natural boron on a Cu backing. Many reaction products are evident (not all are shown in Fig. I), including protons, deuterons, and alpha particles; however, only the highest three peaks are useful for materials analysis under these conditions. These are the p. and p, proton groups from B” and the p. proton group from ’ 'B as indicated in the figure. These high energy proton yields can be accurately measured and calibrated to determine total boron area1 density in unknown samples if the reaction cross section is constant over the range of projectile energies present in the sample. The energy width of these peaks is about 300 keV and is caused by kinematic broadening due to the 16” angular acceptance. Absorber foils could be used with a thinner detector; however, approximately 1.6 mm of Al would be required to slow the 18.8 MeV p. protons gram “B (shown in Fig. 1) to 6 MeV for complete energy deposition in a 300 pm thick detector. We have measured a proton peak width of about 1 MeV for the p. proton group under these conditions (this is considerably higher than predicted by simple Bohr straggling theory). It should be mentioned that alpha particles from the high Q value t3He, a> reactions on B (and N) could be used in conjunction with thin depletion zone detectors to quantify B (and N) as done in Ref. [4]. Figs. 2 and 3 show measured laboratory differential cross sections for the three proton groups mentioned above. Measurements were made in 50 or 100 keV steps between 2 and 4 MeV and at lab angles of 90” and 135”. Natural isotopic abundance of 19.9% “B and 80.1% “B was assumed for the cross section calculations. The B on Cu target was about 40 keV thick at 3 MeV and the energies shown are calculated at the center of the target. The cross sections were calculated from the equation d CJ/ dL! =
L.C. McIntyre Jr. et al./Nucl.
_..*..*.....,.*.**
“WPJ
.**
0.2 0.1 -
.
l.*.
0.11 -
:
.
l.*
‘%PJ
0.07 -
. .
.*..w** “.-
2000
221
Instr. and Meth. in Phys. Res. B 118 (1996) 219-223
l. . .
l ‘..
l. .
tl
. .*.
I&
25ca
3ooo
. l.a
l
=
0
250
350
450
135’
3500
550
650
750
Channel Number
4coQ
Energy (keV) Fig. 3. Measured laboratory differential cross sections for the indicated proton groups from the C3He.p) reaction on the isotopes of boron at a laboratory angle of 135’.
A cos (4 )/( NrQ’(l),
where A is the proton yield, 4 is the angle between the target normal and the incident beam, Nr is the target areal density (atoms/cm2), Q’ is the number of incident particles and D is the detector solid angle. The absolute scale was set by reference to a self supporting natural B foil whose area1 density was measured by RBS assuming a Rutherford scattering cross section at 1300 keV [ 121. The uncertainty in the absolute cross sections is estimated to be IO-15%. mainly due to uncertainty in the reference area1 density and the solid angle. Relative uncertainties varied from 2% to 5% and were determined by counting statistics. These results are consistant with those previously reported for the “B p. and p, groups at 90” [ 131. Our results are about 30% larger for the “B p0 group at 90” than those reported in Ref. [ 141. There is no evidence of resonant behavior, and the nearly constant cross section for the “B p, group between 3200 and 3600 kev incident energy at either 90” or 135” provides a favorable region for determination of total boron in films as thick as 400 keV.
Fig. 4. Partial energy spectrum of reaction products at a laboratory angle of 135” from 3000 keV incident ‘He ions on a TaSiN target on a Si backing. The integrated charge was 20 p.C.
keV under the conditions of Fig. 4. The energy width of these peaks is about 200 keV and is caused by the 16” angular acceptance. Again, absorber foils and a thinner detector could be used. We have measured spectra under the conditions of Fig. 4 using a 360 km Al absorber which reduces the p,-pz proton group to 6 MeV, which can be stopped in a 300 p,m detector, while maintaining separation between the p,-p2 and px-p4 groups. Figs. 5 and 6 show measured laboratory cross sections for these 3 proton groups (or combined groups) between 2 and 4 MeV and at lab angles of 90” and 135”. The TaSiN target used was 40 keV thick for 3 MeV 3He ions. The cross sections were calculated as &scribed in the previous section. The absolute scale was set by reference to a SIN
0.2
“N(PJ t
2.3. Nitrogen Fig. 4 shows a partial energy spectrum of reaction products at 135” from incident 3000 keV 3He+ ions on a TaSiN on Si target containing 5.5 X lOI7 atoms/cm* of nitrogen. As in the boron case, only the three highest energy isolated proton peaks are suitable for materials analysis. These are the 14N po, combined p, and p2, and combined p3 and,?, groups as indicated in Fig. 4. The energy levels in 0 responsible for these doublets have separations of 82 keV (p,-p2) and 200 keV (p3-p4) which results in proton energy separations of 72 keV and 18 1
08
-
l. ...*..
“N(~3,) .**
oi-
....
. .* ..
203
l
.
9
.** 2500
..***
lab
3oQ.l
= grJ’
SOL!
4ooo
Energy (keV) Fig. 5. Measured laboratory differential cross sections for the indicated proton groups from the c3He, p) reaction on nitrogen at a laboratory angle of 90”.
III. NRA
L.C. McIntyre Jr. er al./Nucl.
222
5Yxl
2soo
390
Insrr. and Meth. in Phys. Res. B II8 (1996) 219-223
4cm
Fig. 6. Measured laboratory differential cross sections for the indicated proton groups from the (3He, p) reaction on nitrogen at a laboratory angle of 135”.
Our procedure in the use of these reactions is to run a standard target, whose boron or nitrogen area1 density is known from RBS, and compare the yield to any unknown samples. If this approach is used, computation of cross sections and solid angles is unnecessary as long as care is taken in accounting for any cross section variations due to thickness differences between the standard and unknown. Minimum practical area1 densities which can be measured with our experimental conditions are between 10” and lOi atoms/cm’ for both boron and nitrogen. Whereas a thick (e.g. 1.5 mm) surface barrier detector provides maximum energy resolution for cleanly resolving the useful proton groups, it would be possible to use a thinner (e.g. 300 urn) detector and thick absorber foils for application of this technique. Finally, we should mention that neutron radiation monitoring should be used in laboratories accelerating 3He ions. Ref. [4] has a good discussion of this problem: we also find that the primary source of neutrons is the carbon contamination on beam collimating slits.
Acknowledgements on carbon target whose nitrogen area1 density was measured by RBS. The uncertainty in the absolute cross sections is estimated to be lo-15%, mainly due to uncertainty in the reference areal density and the solid angle. Relative uncertainties varied from 2% to 5% and were determined by counting statistics. The results below 2.8 MeV are consistent with those reported in Ref. [lo] for the pi--p2 and ps-p4 groups, except for the three highest energy cross sections (2577, 2676 and 2775 keV) for the p,-pz at 90” where considerable discrepancies were observed. Other work has been done on this reaction [ 15,161; however, only qualitative comparisons with the present results are possible. The cross sections are all smoothly varying with energy but there are few regions with approximately constant cross section. Perhaps the most favorable regions for materials analysis are the 100 keV region below 3200 keV for the p3-p4 group at 135” or the 200 keV region below 3400 keV for the pe group at 90” or 135”, however the latter group has significantly less yield than the former.
3. Conclusions We find that the high Q value (3He, p) reactions on boron and nitrogen can be useful in determining total boron or nitrogen area1 densities in thin films. However, in many situations there are only marginal advantages over the ((Y, p) reaction for these two elements. We have found that these reactions using incident 3He are not generally suitable for samples on carbon backings. In addition to large yields of low energy protons, large neutron fluxes are produced by the carbon backing.
This work was supported by NSF grant INT-9314133, CONACYT grant 3436-E and a University of Arizona International Program Development Fund Award. The authors would like to thank Dr. J.O. Stoner Jr. for the B on Cu target and R.B. Gregory for use of the TaSiN on Si target. Tabulated values of these measurements will be available on the database SIGMABASE, lhn.gns.cri.nz (13 1.203.40.11, ibaserver.physics.isu.edu (134.50.3.61, http://physics.isu.edu/sigmabase/.
References [l] G. Amsel, J.P. Nadai, E. d’Artemare, D. David, E. Girard and J. Moulin, Nucl. Instr. and Meth. 92 (1971) 481. [Z] C.R. Gossett, Nucl. Instr. and Meth. B 45 (1983) 149. [3] W.N. Lemtard. S.Y. Tong, I.V. Mitchell and G.R. Massoumi, Nucl. Instr. and Meth. B 43 (1989) 187. [4] F. Abel, G. Amsel, E. d’Artemare, C. Ortega, J. Siejka and G. Vizkelethy, Nucl. Instr. and Meth. B 45 (1990) 100. [5] J. Li, G. Vizkelethy, P. Revesz, J.W. Mayer, L.J. Matienzo, F. Emmi, C. Ortega and J. Siejka, Appl. Phys. Lett. 58 (1991) 1344. [6] D.D. Cohen, G.M. Bailey and N. Dytlewski, Nucl. Instr. and Meth. B 64 (1992) 413. [7] A.E. Pillay and M. Peisach, Nucl. Instr. and Meth. B 64 (1992) 465. [8] L.C. McIntyre Jr., J.A. Leavitt, M.D. Ashbaugh, Z. Lin and J.O. Stoner Jr., Nucl. Instr. and Meth. B 66 (1992) 221. [9] Z. Lin, L.C. McIntyre Jr., J.A. Leavitt, M.D. Ashbaugh and R.P. Cox, Nucl. Instr. and Metb. B 79 (1993) 498. [IO] G. Tetwagne, D.D. Cohen and G.A. Collins, Nucl. Instr. and Metb. B 84 (1994) 415.
L.C. McIntyre
Jr. et d./
Nucl. Instr. and Meth. in Phys. Res. B I18 (1996) 219-223
[I I] J.A. Leavitt. Nucl. Instr. and Meth. B 24/25 (1987) 717. 1121 L.C. McIntyre Jr., J.A. Leavitt, M.D. Ashbaugh, Z. Lin and J.O. Stoner Jr., Nucl. Instr. and Meth. B 64 (1992) 457. [131 J.P. Schiffer, T.W. Banner, R.H. Davis and F.W. Presser Jr., Phys. Rev. 104 (1956) 1064. 1141 H.D. Holmgren, E.A. Wohcki and R.L. Johnston, Phys. Rev. 114(1959) 1281.
223
[151 P. Guazzoni, S. Micheletti, M. Piganelli, F. Gentilin and F. Pellegrini, Phys. Rev. C 4 (1971) 1086. 1161 S. Gorodetzky, G. Bassompierre, C. St-Pierre. A. Gallmann and P. Wagner. Nucl. Phys. 43 (1963) 92.
III. NRA
2&k __
Nuclear Instruments and Methods in Physics Research B I I8 (19%) 2 19-223
Rl0NlB
Beam Interactions with Materials A Atoms
ELSEVIER
Cross
section measurements for the ( 3He, p) nuclear reaction on B and N between 2 and 4 MeV
*,
L.C. McIntyre Jr. aV J.A. Leavitt a, M.D. Ashbaugh a, J. Borgardt a, E. Andrade b, J. Rickards b, A. Oliver b a Depurtment ofPhysics. University ofAri:ona. Tucson. AZ 85721, USA b Institute de Fisica, U.NA.M..
Ap. Postal 20364, Mexico. D.F. 01000. Mexico
Abstract Cross section measurements have been made for several proton groups from the t3He,p) nuclear reaction on boron and nitrogen. Incident energies between 2 and 4 MeV and angles of 90” and 135” were used. A 1.5 mm thick surface barrier detector, covered to stop elastically scattered 3He ions, was used to detect the high energy protons from these reactions. The proton energy was as high as 20 MeV in the case of a boron target. We propose the use of these reactions for quantitative elemental areal density determination in thin film analyses.
1. Introduction One of the shortcomings of thin film analysis using Rutherford Backscattering Spectrometry (RBS) with MeV ion beams is the insensitivity to light elements in cases where interfering signals are present due to heavier elements in the film or backing substrate. In many cases the use of nuclear reactions is a viable alternative. Historically the (d, p) reaction is one of the most popular [I]; however, many materials analysis accelerator labs cannot provide adequate radiation shielding for the operating personnel and are reluctant to accelerate deuterons. In some cases, the (3He, p) or c3He, a> reactions can serve as alternatives with a reduced but not negligible radiation hazard. The use of these reactions in the analysis of C and 0 has been reported in several papers [2-71. There are two basic materials analysis techniques using nuclear reactions; one is the use of sharp resonances to depth profile element concentrations, the other is the use of regions of constant cross section to measure total elemental areal densities in thin films. In this paper, we present cross section measurements for several proton groups from the c3He, p) reaction on natural boron and nitrogen using incident energies between 2 and 4 MeV. In all cases the cross sections were found to be smoothly varying with energy: no resonance structure is evident. We have recently reported investigations concerning the use of the (o, p) nuclear reaction for determination of
Corresponding author. Tel. + 1 520 621 6813, fax + I 520 621 4721, e-mail [email protected]. l
B and N in thin films [8,9]. While this reaction can be used successfully in both cases, there are some problems which are not present if the corresponding c3He, p> reaction is IO used. Cross sections for the (a, p) reaction on B, ’ 'B and 14N all show evidence of resonance behavior in the regions explored. However, the resonances are too broad to be used for depth profiling and simply limit the regions of smooth or constant behavior of the cross section. This is particularly true in the case of B [8]. An additional problem with the N (a, p) reaction is the low Q value and possible interference with competing reactions if Si is present [9]. An advantage of the c3He, p) reactions on both boron isotopes and 14N is the large Q value. This results in high energy reaction protons with little chance of overlap by interfering reactions. The Q values for the c3He, p) reaction on “B, “B, and 14N are + 19.7 MeV, + 13.2 MeV, and + 15.2 MeV respectively. The downside is that a detector thick enough to stop these high energy reaction protons is required for optimal results. If a thinner detector is used, these high energy protons will lose some fraction of their energy in the detector and perhaps overlap lower energy particles that have been stopped with complete energy deposition. Thick absorber foils can be used to slow the reaction protons; however, straggling reduces the energy resolution significantly. Since we will not consider depth profiling in this paper, the energy resolution is of concern only for cleanly resolving proton groups for total areal density determination. A recent study was published using the 14N (“He, p) I60 reaction to determine N [IO]; however, only energies below 2.8 MeV were used and the ground state proton
0168.583X/%/$15.00 Copyright 6 1996 Elsevier Science B.V. All rights reserved SSDf 0168-583X(95)01074-2
III. NRA
220
L.C. McIntyre Jr. et al./Nucl. Instr. and Meth.
group was not detected available.
because a thick detector
in
Phyr. Res. B I18 (1996) 219-223
was not
2. Experimental
5
2.1. General
2
‘OB(P,) 0.9 -
E.
Beams of 3He+ with energies between 2 and 4 MeV were obtained from the University of Arizona CN (High Voltage Engineering Carp) Van de Graaff accelerator. A 1.5 mm (nominal) thick Si surface barrier detector with area 300 mm2 (nominal) was positioned approximately 50 mm from the target at laboratory angles of 90” or 135”. The target normal was 45” relative to the beam direction, and the angular acceptance range was + / - 8” about the central angle for both detection angles. The detector was covered with a 23 p,rn thick Mylar foil to stop elastically scattered ‘He+ ions. The nominal thickness of the Ortec surface barrier detector (model BA-023-300-1500) was 1.5 mm; however, since we observe linear behavior for incident protons up to 20 MeV the thickness must be at least 2 mm when operated with the prescribed voltage of 300 V. The solid angle of the detector was determined by a circular collimator and was measured to be 69 +/ - 4 msr. Typical beam currents were 80 nA and a typical run had an integrated charge of 20 KC. This thick detector was added to our usual backscattering setup [ 111:nuclear reaction and elastic scattering data can be taken simultaneously if desired. 2.2. Boron Fig. 1 shows a partial energy spectrum of reaction roducts at a lab angle of 135” from incident 3000 keV P about 2.6 X lOI He+ ions on a target containing
Epw 7
9
11
13
15
17
19
,.,,
@-ie,p)C
I
250
350
I
Ebeam= 3000 keV
I
9 = 135”
450
550
650
750
650
950
Channel Number
Fig, I. Partial energy spectrum of reaction products at a laboratory angle of 135” from 3000 keV incident 3He ions on a natural boron target on a Cu Backing. Tbe integrated charge was 20 KC.
B
0.7 -
;; D
:
2cco
.
0.
_..**.*
.*..-,
.**
2500
30x
3500
4000
Energy (keV)
Fig. 2. Measured laboratory differential cross sections for the indicated proton groups from the c3He, p) reaction on the isotopes of boron at a laboratory angle of 90”.
atoms/cm* of natural boron on a Cu backing. Many reaction products are evident (not all are shown in Fig. I), including protons, deuterons, and alpha particles; however, only the highest three peaks are useful for materials analysis under these conditions. These are the p. and p, proton groups from B” and the p. proton group from ’ 'B as indicated in the figure. These high energy proton yields can be accurately measured and calibrated to determine total boron area1 density in unknown samples if the reaction cross section is constant over the range of projectile energies present in the sample. The energy width of these peaks is about 300 keV and is caused by kinematic broadening due to the 16” angular acceptance. Absorber foils could be used with a thinner detector; however, approximately 1.6 mm of Al would be required to slow the 18.8 MeV p. protons gram “B (shown in Fig. 1) to 6 MeV for complete energy deposition in a 300 pm thick detector. We have measured a proton peak width of about 1 MeV for the p. proton group under these conditions (this is considerably higher than predicted by simple Bohr straggling theory). It should be mentioned that alpha particles from the high Q value t3He, a> reactions on B (and N) could be used in conjunction with thin depletion zone detectors to quantify B (and N) as done in Ref. [4]. Figs. 2 and 3 show measured laboratory differential cross sections for the three proton groups mentioned above. Measurements were made in 50 or 100 keV steps between 2 and 4 MeV and at lab angles of 90” and 135”. Natural isotopic abundance of 19.9% “B and 80.1% “B was assumed for the cross section calculations. The B on Cu target was about 40 keV thick at 3 MeV and the energies shown are calculated at the center of the target. The cross sections were calculated from the equation d CJ/ dL! =
L.C. McIntyre Jr. et al./Nucl.
_..*..*.....,.*.**
“WPJ
.**
0.2 0.1 -
.
l.*.
0.11 -
:
.
l.*
‘%PJ
0.07 -
. .
.*..w** “.-
2000
221
Instr. and Meth. in Phys. Res. B 118 (1996) 219-223
l. . .
l ‘..
l. .
tl
. .*.
I&
25ca
3ooo
. l.a
l
=
0
250
350
450
135’
3500
550
650
750
Channel Number
4coQ
Energy (keV) Fig. 3. Measured laboratory differential cross sections for the indicated proton groups from the C3He.p) reaction on the isotopes of boron at a laboratory angle of 135’.
A cos (4 )/( NrQ’(l),
where A is the proton yield, 4 is the angle between the target normal and the incident beam, Nr is the target areal density (atoms/cm2), Q’ is the number of incident particles and D is the detector solid angle. The absolute scale was set by reference to a self supporting natural B foil whose area1 density was measured by RBS assuming a Rutherford scattering cross section at 1300 keV [ 121. The uncertainty in the absolute cross sections is estimated to be IO-15%. mainly due to uncertainty in the reference area1 density and the solid angle. Relative uncertainties varied from 2% to 5% and were determined by counting statistics. These results are consistant with those previously reported for the “B p. and p, groups at 90” [ 131. Our results are about 30% larger for the “B p0 group at 90” than those reported in Ref. [ 141. There is no evidence of resonant behavior, and the nearly constant cross section for the “B p, group between 3200 and 3600 kev incident energy at either 90” or 135” provides a favorable region for determination of total boron in films as thick as 400 keV.
Fig. 4. Partial energy spectrum of reaction products at a laboratory angle of 135” from 3000 keV incident ‘He ions on a TaSiN target on a Si backing. The integrated charge was 20 p.C.
keV under the conditions of Fig. 4. The energy width of these peaks is about 200 keV and is caused by the 16” angular acceptance. Again, absorber foils and a thinner detector could be used. We have measured spectra under the conditions of Fig. 4 using a 360 km Al absorber which reduces the p,-pz proton group to 6 MeV, which can be stopped in a 300 p,m detector, while maintaining separation between the p,-p2 and px-p4 groups. Figs. 5 and 6 show measured laboratory cross sections for these 3 proton groups (or combined groups) between 2 and 4 MeV and at lab angles of 90” and 135”. The TaSiN target used was 40 keV thick for 3 MeV 3He ions. The cross sections were calculated as &scribed in the previous section. The absolute scale was set by reference to a SIN
0.2
“N(PJ t
2.3. Nitrogen Fig. 4 shows a partial energy spectrum of reaction products at 135” from incident 3000 keV 3He+ ions on a TaSiN on Si target containing 5.5 X lOI7 atoms/cm* of nitrogen. As in the boron case, only the three highest energy isolated proton peaks are suitable for materials analysis. These are the 14N po, combined p, and p2, and combined p3 and,?, groups as indicated in Fig. 4. The energy levels in 0 responsible for these doublets have separations of 82 keV (p,-p2) and 200 keV (p3-p4) which results in proton energy separations of 72 keV and 18 1
08
-
l. ...*..
“N(~3,) .**
oi-
....
. .* ..
203
l
.
9
.** 2500
..***
lab
3oQ.l
= grJ’
SOL!
4ooo
Energy (keV) Fig. 5. Measured laboratory differential cross sections for the indicated proton groups from the c3He, p) reaction on nitrogen at a laboratory angle of 90”.
III. NRA
L.C. McIntyre Jr. er al./Nucl.
222
5Yxl
2soo
390
Insrr. and Meth. in Phys. Res. B II8 (1996) 219-223
4cm
Fig. 6. Measured laboratory differential cross sections for the indicated proton groups from the (3He, p) reaction on nitrogen at a laboratory angle of 135”.
Our procedure in the use of these reactions is to run a standard target, whose boron or nitrogen area1 density is known from RBS, and compare the yield to any unknown samples. If this approach is used, computation of cross sections and solid angles is unnecessary as long as care is taken in accounting for any cross section variations due to thickness differences between the standard and unknown. Minimum practical area1 densities which can be measured with our experimental conditions are between 10” and lOi atoms/cm’ for both boron and nitrogen. Whereas a thick (e.g. 1.5 mm) surface barrier detector provides maximum energy resolution for cleanly resolving the useful proton groups, it would be possible to use a thinner (e.g. 300 urn) detector and thick absorber foils for application of this technique. Finally, we should mention that neutron radiation monitoring should be used in laboratories accelerating 3He ions. Ref. [4] has a good discussion of this problem: we also find that the primary source of neutrons is the carbon contamination on beam collimating slits.
Acknowledgements on carbon target whose nitrogen area1 density was measured by RBS. The uncertainty in the absolute cross sections is estimated to be lo-15%, mainly due to uncertainty in the reference areal density and the solid angle. Relative uncertainties varied from 2% to 5% and were determined by counting statistics. The results below 2.8 MeV are consistent with those reported in Ref. [lo] for the pi--p2 and ps-p4 groups, except for the three highest energy cross sections (2577, 2676 and 2775 keV) for the p,-pz at 90” where considerable discrepancies were observed. Other work has been done on this reaction [ 15,161; however, only qualitative comparisons with the present results are possible. The cross sections are all smoothly varying with energy but there are few regions with approximately constant cross section. Perhaps the most favorable regions for materials analysis are the 100 keV region below 3200 keV for the p3-p4 group at 135” or the 200 keV region below 3400 keV for the pe group at 90” or 135”, however the latter group has significantly less yield than the former.
3. Conclusions We find that the high Q value (3He, p) reactions on boron and nitrogen can be useful in determining total boron or nitrogen area1 densities in thin films. However, in many situations there are only marginal advantages over the ((Y, p) reaction for these two elements. We have found that these reactions using incident 3He are not generally suitable for samples on carbon backings. In addition to large yields of low energy protons, large neutron fluxes are produced by the carbon backing.
This work was supported by NSF grant INT-9314133, CONACYT grant 3436-E and a University of Arizona International Program Development Fund Award. The authors would like to thank Dr. J.O. Stoner Jr. for the B on Cu target and R.B. Gregory for use of the TaSiN on Si target. Tabulated values of these measurements will be available on the database SIGMABASE, lhn.gns.cri.nz (13 1.203.40.11, ibaserver.physics.isu.edu (134.50.3.61, http://physics.isu.edu/sigmabase/.
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III. NRA